The purpose of the research is to provide a detailed morphologic analysis of the development of the rabbit testis during the postnatal prepubertal period. The study will include: (1) transmission electron microscopy observations on interstitial cell development following HCG stimulation; (2) freeze fracture and lanthanum tracer studies on the development of the blood-testis barrier; (3) scanning electron microscopy observations on the relationships between Sertoli cells and germ cells in the prespermatogenic tubules; (4) localization of cholesterol ester hydrolysis activity in the different compartments of the developing testis. Testicular specimens from birth to 15 weeks of age. Blood samples will also be collected to determine levels of testosterone and other androgens following HCG administration.